Take a waiting list link in events' front matters into account:
- An additional waiting list link can be given, which will be rendered on the event's page and events' overview list.
- There are two new cases:
- Event is booked out, there is no waiting list. => Display badge "fully booked out".
- Event is booked out, but there is a waiting list. => Display waiting list button.
Closes #140 (closed)